Benzene is a toxic carcinogenic chemical. Benzene concentration in gasoline blends is placed under many environmental regulations worldwide. The Mobile Source Air Toxics (MSAT) II regulations that became effective from Jan. 1, 2011 restricts the annual average benzene level in gasoline sold in US to 0.62% vol. Typically reformate, hydrogenated pyrolysis gasoline (PG) and catalytically cracked gasoline are the main contributors of benzene in the gasoline pool.
Contrary to this, Benzene is also used extensively as an intermediate to make a variety of industrial chemicals. About 80% of benzene is consumed in the production of three chemicals, ethylbenzene, cumene, and cyclohexane. Its most widely produced derivative is ethylbenzene, precursor to styrene, which is used to make polymers and plastics. Cumene is converted phenol for resins and adhesives. Cyclohexane is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives, and pesticides.
FCC Gasoline as an Alternative Feedstock for Benzene Production
Currently FCC gasoline comprises nearly 10-20% of the gasoline pool in a typical refinery. In the near future with increase in demand for lighter fuels, polymer industry pre-cursors like ethylene and propylene, FCCs will be run at higher severities so as to extract the most from heavy residuals and gas oils. High-severity FCC is thus intended to increase olefin yields. Propylene yields can be increased from 3-5% in conventional FCC to 15-28% in the high-severity process. In a high-severity FCC operation, the aromatic content of cracked naphtha is also increased to the level of 50-70% but this contains significant amounts of thiophenic sulphur impurities and is high in olefin content.
Full range FCC gasoline contains around 15-30 vol. % aromatics with up to 2 vol. % benzene and 1000-2000 ppm sulfur. A narrow C6 heart cut fraction of the full range gasoline will contain anywhere between 11-19 wt. % benzene and up to 500 ppm sulfur. So it becomes crucial to establish technologies not only to process the FCC gasoline for efficient recovery of high value materials (like benzene) but also to render it market ready (in terms of low sulfur and benzene content).
Recovery of benzene from FCC gasoline is comparatively less straight forward. The relationship of feed properties and reaction process conditions to the production of various compounds in a FCC unit is complex and thus does not present a straight forward solution for benzene control. Unlike reformate and hydrogenated PyGas, unprocessed cracked gasoline fraction (from FCC or Thermal Crackers) contain olefins along with impurities like oxygenates, metals, chlorides, sulphur compounds, nitrogen compounds, and organic peroxides. Due to the complex nature of this feedstock, an economic and reliable benzene recovery process is difficult to develop and has not been practiced in the industry so far.
Technologies being used worldwide have the sole purpose of either recovering aromatics or reducing aromatics from petroleum feedstocks like reformate, pyrolysis gasoline, or cracked gasoline fractions. There are no technologies in operation worldwide which serve the dual purpose of producing benzene lean streams from a petroleum feedstock (like the ones mentioned above), by simultaneously recovering high purity benzene.
Well known hydroprocessing routes aimed at reducing benzene from olefinic feedstocks like cracked gasoline result in saturating the olefins thus lowering the octane of the cracked gasoline fraction. Olefins in cracked gasoline contribute substantially to the octane in the gasoline pool. An attempt to reduce benzene by well-known hydro-processing routes would result in saturating the olefins as well, thus lowering the octane of the cracked gasoline fraction.
In hydrocarbon industries, aromatics like benzene, toluene, xylenes (BTX) are mainly recovered from reformate and hydrogenated pyrolysis gasoline using liquid-liquid extraction (LLE) or extractive distillation (ED) using polar aromatics selective solvents like NMP, Sulfolane, NFM, etc. There are many commercial units worldwide based on LLE and ED which are in operation and process the above mentioned feedstocks to produce BTX.
A number of patents are available which describe these processes. For example U.S. Pat. No. 3,591,490 highlights an extractive distillation process for recovery of xylenes and ethyl benzene from hydrogenated feedstocks like hydrogenated pyrolysis gasoline fraction using N-Methyl-2-Pyrrolidone (NMP) or Di-methylformamide (DMF) as a solvent. The process described comprises processing the feedstock in an extractive distillation unit followed by treating the raffinate in a washing column (a countercurrent extractor) in presence of extra circulating solvent and washing water. The solvent rich product from the extractive distillation column is then taken to a stripper column or the solvent recovery column where separation between the dissolved aromatic hydrocarbons and the solvent is affected.
Similarly U.S. Pat. No. 3,723,256 describes as process to recover BTX rich aromatic streams from a hydrotreated C6-C8 pyrolysis naphtha cut (containing about 18% non-aromatics) using a combination of pre-fractionation, extractive distillation and solvent extraction with sulfolane. U.S. Pat. No. 5,022,981 describes a solvent extraction based process for recovery of high purity aromatics from a C6-C9 feed stream. The typical feedstocks suitable for the process are; the liquid by-product stream from a pyrolysis gasoline unit after being hydrotreated or the product of a catalytic reforming unit. The solvent employed in the process is mixture of tetraethylene glycol and methoxytriglycol.
U.S. Pat. No. 7,501,549 describes a complex integrated process of benzene removal from FCC Naphtha. The main steps comprise fractionating the FCC Naphtha to obtain a benzene concentrate stream. The benzene concentrate stream is subjected to etherification over a catalyst to convert C6 Iso-olefins in the feed to corresponding ethers. This is followed by fractionation to separate the ethers from the remaining C6 fraction stream. The remaining C6 stream is then hydrotreated and can then be processed in a solvent extraction unit using Triethylene Glycol or Sulfolane.
U.S. Pat. No. 8,143,466 highlights a process for removal of benzene from reformate FCC gasoline, coker pentane/hexane, coker naphtha, FCC naphtha, straight run gasoline, pyrolysis gasoline, coal oven naphtha, and mixtures containing two or more of these streams. The benzene removal is affected by alkylation of the benzene rich feedstock wherein the benzene in feed is catalytically alkylated with alcohol and ethers to higher aromatics like toluene in a reactive distillation column and the products are separated.
U.S. Pat. No. 8,722,952 B2 elaborates an extractive distillation process scheme for production of benzene lean Gasoline by recovery of high purity benzene from unprocessed cracked gasoline fraction containing organic peroxides. The invention incorporates an intricate process involving NMP+Water solvent system. All the columns (Extractive Distillation Column, Raffinate Section Stripper, Solvent Recovery Column, and Extract Section Stripper) of the process operate above atmospheric pressure. Benzene recovery in the final extract product is more than 99% of the feed benzene with a purity of more than 97 wt. %. The simultaneous benzene content in the US Grade Gasoline obtained as the raffinate is <0.4 wt. %. The total hot and cold utility requirement for the process (for 70 tph feed throughput) is about 23.76 MMKCal/hr and 25.73 MMKCal/hr respectively.
Any process for simultaneous production of high purity benzene and US Grade Gasoline from cracked gasoline fraction which can increase the purity of benzene to reduce the cost of secondary processes (which utilize benzene) with reduction in capital investment and operating cost is of great importance.
The current specification highlights an energy efficient and cost effective (vacuum based extractive distillation) improved process over the process scheme described in U.S. Pat. No. 8,722,952 B2 with same final product specifications. Improvement with respect to increase in product yields and purities, significant reduction in hot and cold utility requirement by more than 33% and 31% respectively and considerable reduction in capital investment by elimination of raffinate and extract sections strippers and associated machinery.